1. Field of the Invention
The present invention relates to an aluminum alloy which shows low friction characteristics. It is suitable for use as engine components of automobiles and is excellent in both tensile strength and wear resistance.
2. Description of the Related Art
An aluminum alloy has light weight and excellent processability. So it has been conventionally used as structural materials of air planes and automobiles. Recently, an engine of automobiles comes to require high power and low fuel consumption. In accordance with this requirement, the aluminum alloy is being applied for rocker arms, shift forks and engine components such as piston or cylinder head. So, the aluminum alloy is improved in its wear resistance and tensile strength.
Al-based composite materials having excellent wear resistance and excellent stiffness include, for example, a high tensile aluminum alloy material. It is produced by powder metallurgy in which particles, whiskers and fibers of SiC or Al.sub.2 O.sub.3 are added into Al--Cu--Mg alloy (2000 series) or Al--Mg--Bi alloy (6000 series).
A high tensile aluminum alloy powder having excellent tensile strength, excellent wear resistance and low thermal expansion is developed (See Japanese Patent Publication No. 56401/1990). The method for producing the high tensile aluminum alloy powder is that 7.7 to 15% of Ni is added to an Al--Si alloy, then Cu and Mg are added. Concerning the obtained high tensile aluminum alloy powder, the size of primary Si is less than 15 .mu.m.
Regarding piston, a skirt portion requires excellent wear resistance, excellent heat conductivity, low thermal expansion and excellent tensile strength. Cylinder liner requires excellent wear resistance, excellent antiseize and low friction coefficient.
The above alloy such as 2000 series alloy or 6000 series alloy is used as matrix, and particles, whiskers and fibers of SiC or Al.sub.2 O.sub.3 are added into the matrix, thereby obtaining Al-based Metal Matrix Composites (hereinafter described as MMC). It shows poor tensile strength because the matrix itself shows poor tensile strength.
When the above Al-based MMC is used as a sliding member of the above piston or the above cylinder liner, the temperature of a sliding portion rises. So, agglutination abrasion or abrasive friction generates, and friction coefficient becomes high and abrasion loss becomes large. Therefore, to use the Al-based MMC as the sliding member is restricted not only at high temperature but also room temperature.
The above high tensile aluminum alloy in which Ni is added into an Al--Si alloy shows excellent tensile strength because stable Al--Ni intermetallic compounds are formed. When the high tensile aluminum alloy is used as a sliding member, it shows poor wear resistance since hard particles such as ceramics are not included. Concerning sliding characteristics, Al is adhered to the mating member because of agglutination. The high tensile aluminum alloy cannot be improved in its friction coefficient, seize load and abrasion loss. Therefore, the high tensile aluminum alloy is used as the sliding member only for the restricted area under the restricted condition.
When the conventional aluminum alloy is used as the sliding member of the engine component, it shows poor tensile strength and poor sliding characteristics.